The present invention relates, in general, to a current source with an adjustable temperature coefficient, and more particularly to a current source suitable for supplying a temperature compensated current to a load including, but not limited to an electro-optical device or system.
Many electro-optical systems require a constant light output intensity however the light output intensity will cause undesired performance changes in the system unless temperature compensation is used. Even if there is no other reason, temperature compensation of electro-optical devices is desirable to avoid excessive power dissipation in the optical source and its associated circuitry. In some systems the current required to ensure reliable operation at the lowest temperature will provide excessive current at the highest temperature causing further heating and eventual failure of the device.
Temperature compensation in most electronic circuits is achieved by means of electrical feedback from the ultimate output in some fashion. Electro-optical devices are among the devices in which electrical feedback is difficult to achieve, making such temperature compensation schemes impractical. Optically isolated devices are particularly difficult to compensate for temperature since they consist of a plurality of components with differing temperature characteristics and in addition they involve two or more independent electrical circuits which are often sourced from different power supplies operating at different potentials. In the past, temperature compensation of optically isolated devices has been limited to selection of external current limiting components having inherent temperature variations which compensate to some degree for the temperature variation of the optically isolated device and by accepting a reduced temperature range for operation.
In addition, many applications may also operate over a wide range of voltages which implies a need for a low voltage drop through the current source. For example, a solid state relay application must operate with voltages ranging from 3 to 32 volts. In order for the relay to operate at the low end of this range, a current source must have a voltage drop of 1.5 volts or less.